Pneumosinus dilatans is a generalized or partial enlargement of the paranasal sinuses containing only air. Pneumosinus dilatans occurs as an idiopathic disorder as well as in association with other disorders, including cerebral hemiatrophy. We report two cases of patients with congenital cerebral hemiatrophy who developed juvenile pneumosinus dilatans of the frontal, ethmoidal, and sphenoidal sinus. The hydrocephalus of both patients was treated by prolonged cerebrospinal fluid shunting. The development of hyperpneumatization of the paranasal sinuses was proved by plain radiographs and CT. Previous reports of pneumosinus dilatans are reviewed, and the effect of prolonged cerebrospinal fluid shunting in our cases is discussed. Cerebrospinal fluid shunting during the period of physiological pneumatization of paranasal sinuses might have increased hyperpneumatization.